This capacitive sensor that operates with a high frequency oscillator was found in Radio Electronics from the 90's. The circuit operates around 300 kHz.
E) We found this circuit in a PE Hobbyst Handbook from the 90s. The components are common, except...
Found in 1967 documentation, this circuit makes use of period transistors and drives a common...
This circuit can be made usng the TIP32 transistor but must use carbon microphone (not very common these...